Grease a 13x9 baking pan with non-stick cooking spray and a dusting of flour.
In a large mixing bowl, whisk together the flour, sugar, baking soda, salt and cinnamon.
In a small saucepan, combine cola, 1 cup butter and cocoa powder. Bring just to a boil so butter is melted, stirring occasionally. Remove from heat and cool slightly.
Add cola mixture to the flour mixture and stir just until moistened.
In a small bowl, whisk the eggs, buttermilk and vanilla extract until well combined.
Add to the flour mixture and whisk until well combined. Pour into the prepared pan.
Bake for 30-35 minutes or until a toothpick inserted in the center of the cake comes out clean.
Glaze Instructions
While the cake is baking, sift the powdered sugar and cocoa and set aside.
In a medium saucepan, bring the cola to a boil and continue cooking until the liquid is reduced to ½ cup, about 12-15 minutes.
Stir in the butter and cocoa powder and continue stirring until butter is melted. Remove from heat.
Whisk in powdered sugar a little at a time, stirring until the glaze is smooth.
Pour glaze over the warm cake. Let the cake sit on the counter until the glazed has firmed up.
Notes
Don't skip sifting the powdered sugar and cocoa - it makes the glaze smoother.
Can be made in a sheet pan but you’ll need to adjust the baking time.
Leftovers should be stored in an airtight container on the counter or in the refrigerator.
You can adjust the serving sizes to 12 large pieces or 20 smaller pieces of cake.